Method for producing hydrogen from water by means of a  high-temperature electrolyzer

ABSTRACT

Water is heated to a process temperature of an electrolyzer, e.g., more than 500° C. Then, the water is electrolyzed in the electrolyzer to form product gases hydrogen (H 2 ) and oxygen (O 2 ). The product gas hydrogen (H 2 ) is compressed by a compressing apparatus and cooled by a cooling medium that feeds the thermal energy to heat the water.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national stage of International Application No. PCT/EP2011/054768, filed and claims the benefit thereof. The International Application claims the benefits of German Application No. 102010020265.7 filed on May 11, 2010, both applications are incorporated by reference herein in their entirety.

BACKGROUND

Described below is a method for producing hydrogen from water using a high-temperature electrolyzer and a device for producing hydrogen that includes a high-temperature electrolyzer.

Storage of large amounts of electrical energy from renewable resources (e.g. wind energy, solar power) is necessary owing to the rapid development of renewable energies. This includes both short-term backup because of the daily fluctuation, and long-term storage on account of the seasonal variations of renewable energy production and of the demand for electric power. When electrical energy is stored using high-temperature electrolysis of steam, which is advantageous on account of the low electrolysis cell voltage, additional energy must be expended for evaporating liquid water and for compressing the gaseous products hydrogen and oxygen. This energy expenditure must be minimized, in order to maximize the efficiency in storage of electrical energy.

If storage of electrical energy is carried out by high-temperature electrolysis of steam, until now an evaporator for liquid water has been used at system input and compressors for the gaseous products at system output. In this case the evaporator must be supplied with the enthalpy of evaporation for the required educt steam and the compressors must be supplied with the compression energy for the gaseous products hydrogen and oxygen.

SUMMARY

Therefore the problem to be solved is to provide a device, and a method by which water is converted to hydrogen by high-temperature electrolysis, which has a markedly improved energy balance compared to the related art.

The method for producing hydrogen from water by a high-temperature electrolyzer includes the following:

First the water is heated to a process temperature of the electrolyzer, wherein the process temperature is as a rule above 500° C., in particular between 600° C. and 800° C. Next, electrolysis of the water takes place in the electrolyzer, with formation of the gaseous products hydrogen and oxygen from the educt, water. Then the gaseous product hydrogen is compressed with a compressing device and the compressed hydrogen is cooled by a coolant. The heat energy that accumulated in the coolant during cooling is supplied to the heating process of the water that forms the educt of the process.

Compression of the resultant gaseous product hydrogen is necessary or desirable for interim storage of the hydrogen in the smallest possible volume, until it is used again for obtaining energy. However, the compression introduces work into the gaseous product hydrogen (H2). Owing to this work of compression that is introduced, the hydrogen is heated strongly, so that at a pressure that is for example 100 bar after compression, depending on the external boundary conditions, it can have temperatures above 400° C. This energy in the form of heat of the hydrogen is transferred to a coolant, so that the coolant is heated and for its part now incorporates the heat energy of the now cooled hydrogen. Advantageously, this heat energy is used again for heating the water that forms the starting substance, i.e. the educt of the electrolysis process. In this way the energy balance of the complete process is improved markedly.

The absorption of the heat energy by a coolant can also be called a heat exchange process. To make the heat exchange process as efficient as possible, it may be desirable to design the compression process and the associated heat exchange or cooling process in several stages, i.e. in a cascade.

In the heat exchange process, it is desirable for the compressing device, for example a compressor, already to be cooled by the coolant. The heat exchange device is therefore an integral component of the compressing device.

On the other hand it may also be desirable to insulate the compressing device from its surroundings at least partially in the zones with the greatest temperature loading, and to cool the compressed gas for example in the subsequent gas pipeline by a heat exchange device.

Basically it is desirable in all alternatives to have cooling channels passing through the compressing device, so that the heat that accumulates in the compressing device can already be used advantageously.

In another desirable configuration, the coolant that cools the compressed gaseous product hydrogen and is part of the heat exchange device is included in the water that once again represents the educt for the electrolysis process. In this case it is actually the educt water (process water) that is led through the heat exchange devices as coolant and finally is fed in evaporated form into the electrolyzer at the corresponding temperature.

It has also proved desirable to cool the hydrogen leaving the electrolyzer, which also has the process temperature of the electrolyzer of from approx. 500° C. to 800° C., before it is compressed. This cooling operation before compression is desirable to avoid unnecessary thermal loading of the compression device. It is then desirable for the hot gaseous product hydrogen and/or the hot gaseous product oxygen to enter a heat exchange process with the educt water in the form of vapor. Through this heat exchange process, which may also be applied in the form of a countercurrent heat exchanger, the waste heat of the gaseous products can be used directly for heating the educt. With particularly good insulation of the electrolyzer itself and with efficient heat exchange of the products and educts, the energy loss of the high-temperature electrolyzer is extremely low.

In the method described so far, in particular the hydrogen is compressed as product gas and the resultant energy is returned to the water heating process. Basically it may also be desirable to compress the oxygen, with interim storage thereof in a specially provided tank. If the oxygen is also compressed, the resultant heat of compression can also be led away similarly to the above description and utilized for heating the water, which further increases the efficiency of the process.

Also described below is a device for producing hydrogen. This device includes a high-temperature electrolyzer for converting water to hydrogen. The device further includes a compressing device for compressing the hydrogen produced. The compressing device includes a heat exchange device, which serves for transferring the heat of compression that arises in the compression process, to the educt water of the high-temperature electrolyzer. The compressing device is may be a compressor.

It may be desirable for the compressing device to be insulated from its surroundings, especially in the zones with the greatest temperature loading. In this case the compressing device does not give up the resultant heat of compression to the surroundings; the resultant heat of compression can be extracted from the hydrogen by a downstream heat exchange device after the compression process.

Alternatively or additionally, it may be desirable for the heat exchange device to partially surround the compressing device, for example with cooling channels passing through the compressing device and therefore extracting the heat energy introduced via the compression unit from the compressing device.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and advantages will become more apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings of which:

FIG. 1 is a schematic block diagram illustrating a sequence of hydrogen production by a high-temperature electrolyzer with subsequent compression of the hydrogen,

FIG. 2 is a schematic block diagram illustrating a process sequence as in FIG. 1, where a coolant is educt water,

FIG. 3 is a schematic view of a compression device with cooling channels passing through it, and

FIG. 4 is a schematic view of a compression device with insulation and a downstream heat exchanger.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.

In the following, the sequence of the method for producing hydrogen from water by high-temperature electrolysis is explained schematically based on FIG. 1. First, liquid water, designated by H2Of, is led via a water feed pipe 18 into a heat exchanger 10. In this heat exchanger 10 there is already a heated coolant 6, the heating of which will be discussed further later, by which the liquid water is heated to a temperature in the region of the boiling point and is evaporated.

Next, the evaporated water, designated H2Og, i.e. gaseous water, is fed into another heat exchanger 10′. This heat exchanger 10′ heats the steam or the gaseous water H2Og roughly to the process temperature of the high-temperature electrolyzer 2. The gaseous water H2Og, which is led into the high-temperature electrolyzer 2, has a temperature from approx. 500 to 800° C. In the electrolyzer 2, which is also designated as solid oxide electrolyzer (SOE), the water H2Og is converted in a manner known per se into the gaseous products hydrogen (H2) and oxygen (O2). The gaseous products, of which in particular the hydrogen is dealt with hereafter, leave the electrolyzer 2 and also have roughly the process temperature of the electrolyzer. The gaseous products, especially the hydrogen, are once again cooled in a heat exchanger 10′ to a temperature that may be below 100° C., such as about room temperature or elevated room temperature (approx. 50° C.).

The two heat exchangers 10′ are, purely for graphical reasons, shown in FIG. 1 in one circuit, but at separate places. A so-called countercurrent heat exchanger may be employed for heat exchanger 10′, wherein on the one hand the relatively cold coolant, in this case the gaseous water, and on the other hand the hot medium, in this case the hydrogen leaving the electrolyzer, are fed into two intertwined channels, wherein these two media are heated mutually, although separated from one another by the system. This means that the hot steam at about 100° enters on one side of the countercurrent heat exchanger 14, where it meets the hot hydrogen at about 600 to 800° C. In the countercurrent flow of this heat exchanger, the hydrogen H2 is cooled more and more, until it roughly has the starting temperature of the incoming steam, i.e. roughly 100° C. Correspondingly, as it passes through the countercurrent heat exchanger 14, the steam H2Og is heated more and more strongly, until at the outlet from the heat exchanger 14 it has almost the temperature of the hydrogen H2 leaving the electrolyzer 2.

Certainly there are some heat losses with this heat exchanger, but these are relatively slight with good insulation of the complete system, i.e. of the incoming and outgoing gases and the electrolyzer 2, once the complete system is heated to the process temperature between 600° C. and 800° C. The energy loss of the high-temperature electrolyzer is thus relatively slight owing to the heat exchange process described above and good insulation of the complete device.

It should be pointed out here that for process engineering reasons, small amounts of hydrogen, which is tapped from the outgoing product hydrogen, are supplied to the educt water.

The hydrogen, now cooled to a temperature around 100°, which can subsequently be cooled even further or can be cooled with another heat exchanger, must subsequently be compressed for economical storage. It may be advantageous to carry out compression of the hydrogen to about 100 bar. With 100-fold pressurized storage relative to atmospheric pressure, the volume of the gas is reduced 100-fold. The aim is to store the hydrogen in a storage tank with maximum saving of space. A storage pressure of approx. 100 bar (especially between 50 bar and 200 bar) has proved technically advantageous and can be achieved economically.

However, on compression of the hydrogen, work of compression is introduced into the gas, i.e. hydrogen (or with similar application, also into the second product gas oxygen). In the compressed gas system, this work of compression is converted into heat, so that the compressed hydrogen is heated strongly. Depending on the external boundary conditions, at a pressure of usually 100 bar the compressed hydrogen has a temperature from 200 to 600° C. Essentially, this heat energy of the gas is the energy that was previously introduced in the form of electrical energy into the compressor and would be lost, if the gas, as usually occurs, were to be cooled. This energy would be given up to the surroundings. This, however, is an uneconomical process, so that at this point, advantageously, a heat exchange process is also introduced, which extracts the heat energy from the compressed gas and this extracted heat energy is once again supplied to the educt water, in particular the liquid water. This is represented in FIG. 1 by the two heat exchangers 10 at the start of the process and at the end of the process. Heat exchanger 10 at the bottom of FIG. 1, i.e. in the region of the liquid water H2Of, and heat exchanger 10 at the top of FIG. 1 in the region of the hydrogen H2 that is to be compressed, constitute a single unit, they are joined together by lines 7, which transport the corresponding coolant 6, and are therefore given a single reference symbol.

The heat exchanger 10 shown at the top includes a compression device 4, which can, for example as shown in FIGS. 3 and 4, be arranged in the form of a compressor 12, and extracts the heat energy from the compression device 4. This can take place in [[a]] cooling, but it can also take place in cascade fashion, by connecting several compression devices 4 with heat exchange devices 10 one after another, as shown for example in FIG. 1.

The finally compressed product gas, here for example hydrogen, is now stored in compressed form in a tank 16 shown schematically. On the right-hand side, next to the flow of the hydrogen, for example the outlet for oxygen O2 is also shown, which usually is not compressed, as storage of oxygen is only worthwhile in a few cases, although basically it would also be possible to proceed with the oxygen as with the hydrogen, which would make the energy balance of the heat exchanger even more efficient in the system described above.

In the process described in FIG. 1, the coolant 6 flows in each case in a closed circuit, but the process medium, i.e. water, takes a different path, entering the electrolyzer 2 as educt and being converted to hydrogen. An alternative to this is shown in FIG. 2, with the coolant 6 arranged in the form of the process water. At top right of FIG. 2, the liquid water, designated by H2Of, is fed into the cooling-line system of the cooling lines 7 as coolant 6, it flows through the heat exchanger 10, which extracts the heat from the compressed hydrogen. The water is already heated, e.g., to a temperature above 100°, from now on it is in a gaseous state and is brought along line 7 into the next heat exchanger 10′, which once again, in a favorable configuration, can be arranged in the form of a countercurrent heat exchanger. In this case the water (H2Og) is confronted with the very hot product gas H2 (>500° C.) and exchanges the heat contents with this. After flowing through this heat exchanger 10′ or 14, the gaseous water, now already preheated roughly to the process temperature of about 600° to 800° C., is fed into the electrolyzer 2 and is then converted to hydrogen in a manner known per se. Of course, additional thermal heating can also take place, to compensate for heat losses to the surroundings.

The hydrogen leaves the electrolyzer, is cooled in heat exchanger 14 as described, then once again reaches temperatures between room temperature and 100° and is compressed by the compression device 4. After that, in this example according to FIG. 2, otherwise than in FIG. 1, the compression device is not cooled directly, but, as will be mentioned later, is insulated, so that the hot hydrogen is subsequently cooled in the heat exchanger 10 as described. A further cascade of compression can take place in another compression device 4, after which the now completely compressed hydrogen is once again stored in a corresponding tank 16. Insulation of the compression device and subsequent heat exchange is of course also applicable to the embodiment in FIG. 1.

FIGS. 3 and 4 show, as examples, schematic representations of a compression device 4, which in each case have a compression chamber 20 and a reciprocating piston 26. The compression process is shown very schematically and FIGS. 3 and 4 make no claim to completeness. Both compressors 12 in FIG. 3 and FIG. 4 have in common that they have both a hydrogen inlet 22 and a hydrogen outlet 23. The reciprocating piston 26 is moved up and down in the compression chamber 20, which is shown by the double arrow alongside the reciprocating piston. The reciprocating piston 26 may be driven by a cam device, for example by a camshaft.

In FIG. 3, cooling channels 8 pass through the compressor 12, i.e. a special configuration of the general compressing device 4. These cooling channels 8 take up the heat generated in the compression process and lead it away. The cooling channels 8 are therefore a component of the heat exchange device 10 in a concrete configuration.

In the configuration according to FIG. 4, the compressing device 4 or the compressor 12 is provided with insulation 28, wherein a hydrogen inlet 22 of a hydrogen outlet 23 is also provided, the compressor 12 for its part operates at an increased process temperature and the hydrogen, which leaves line 23 already in compressed form, is now cooled by a separate heat exchanger 10. The heat energy that is extracted is used for heating the process water, as already described. Of course, the compression device 4 from FIG. 3 can also have insulation, so that the heat energy produced is led away in concentrated form by the cooling channels 8. 

1-12. (canceled)
 13. A method for producing hydrogen from water using an electrolyzer, comprising: heating the water to a process temperature of the electrolyzer of more than 500° C.; performing electrolysis of the water in the electrolyzer to produce gaseous hydrogen and oxygen; compressing the gaseous hydrogen by a compressing device to produce compressed hydrogen; cooling the compressed hydrogen by a coolant, wherein said heating includes supplying heat energy absorbed by the coolant to said heating of the water which is thereby evaporated, and cooling the gaseous hydrogen, after leaving the electrolyzer and before compression, by a heat-exchange process that heats the water fed to the electrolyzer in vapor form and cools the gaseous hydrogen.
 14. The method as claimed in claim 13, wherein said compressing of the gaseous hydrogen takes place in several compression operations, respectively accompanied by several cooling processes.
 15. The method as claimed in claim 14, wherein the compressing device is cooled by the coolant, whereby the heat energy transferred to the coolant is supplied for said heating of the water.
 16. The method as claimed in claim 15, wherein the compressing device is insulated at least partially from surroundings.
 17. The method as claimed in claim 16, wherein cooling channels pass through the compressing device.
 18. The method as claimed in claim 17, wherein the water that is fed to the electrolyzer includes at least part of the coolant.
 19. The method as claimed in claim 18, wherein after leaving the electrolyzer, the gaseous hydrogen is cooled by a countercurrent heat exchanger.
 20. The method as claimed in claim 19, further comprising, in addition to the gaseous hydrogen, compressing and cooling the oxygen produced by the electrolyzer, with the heat energy thereby extracted being supplied for said heating of the water.
 21. A device for producing hydrogen, comprising: a high-temperature electrolyzer converting water to hydrogen; a compressing device compressing the hydrogen produced by said high-temperature electrolyzer and including a heat exchange device, at least partially surrounding a compression chamber of said compressing device, transferring heat produced by compressing the hydrogen to the water fed to said high-temperature electrolyzer.
 22. The device as claimed in claim 21, wherein said compressing device is a compressor.
 23. The device as claimed in claim 22, wherein said compressor is at least partially insulated.
 24. The device as claimed in claim 23, wherein the heat exchange device includes cooling channels passing through said compressing device. 